Metallic bi-polar plates used in fuel cells are subject to corrosion. The reasons for corrosion are acidic environment, due to Polymer electrolyte membrane (PEM) leaching in PEM fuel cells; galvanic corrosion mechanism, due to galvanic coupling of dissimilar materials, especially graphite and metal; oxidizing or reducing environment; and elevated temperature in the fuel cell. This corrosion, combined with extremely long service times which are required in order to make fuel cells technically and commercially viable, prevents successful commercialization of the fuel cell technology. There is a need to develop bi-polar plates which can withstand an aggressive environment inside the fuel cell over a long period of time, at least 1-3 years and preferably more, are easily manufacturable, and are made of inexpensive materials.
The two main approaches to the materials selection for bi-polar plates are metallic bi-polar plates and graphite-based, or graphite composite-based, bi-polar plates.
The metallic bi-polar plates can be made of inherently corrosion-resistant materials such as titanium or titanium alloys. The main disadvantage of this approach is that these materials are very expensive and difficult to process during manufacturing. In addition, corrosion-resistant metals and alloys, such as titanium, tend to form thick non-conductive passive layers and thus increase the surface resistance of the metal, resulting in poor overall conductivity of the bi-polar plate and increased ohmic losses in the fuel cell. To overcome this problem, techniques preventing surface passivation can be used, such as noble metal deposition, or titanium surface modification or treatment, such as nitriding, to form non-passivating, conductive titanium nitride. These surface treatments are difficult to apply and are expensive.
Another type of metallic bi-polar plate is the plate made of corrosion-resistant material, such as stainless steel, but additionally protected from corrosion by anti-corrosion coatings. In this case, to diminish corrosion of the plate, the steel is coated with titanium nitride or plated with gold or other noble metal. These coatings are difficult to apply and are very expensive.
Yet another technique is electroplating the metallic bi-polar plate with a coating, such as chrome-based or nickel-based electroplated coating. These coatings are not able to withstand conditions in the fuel cell over extended periods of time, and eventually corrode.
Another type of coating which can be employed is conductive composite coating, or conductive paint, typically consisting of organic binder and fine conductive powder such as graphite or metal powder. These coatings have insufficient conductivity. This results in poor overall conductivity of the bi-polar plate and increased ohmic losses in the fuel cell. In order to increase conductivity of such coatings, the loading of the conductive powder should be as high as possible. However, this results in poor coating properties, such as very high viscosity, poor adhesion, susceptibility to delaminantion and cracking of the coating, poor thermal cycling resistance, and poor corrosion resistance. On the other hand, improving coating properties such as viscosity, adhesion, delaminantion, cracking, thermal cycling resistance, and anti-corrosive properties requires higher concentrations of the binder in the coating, which results in poor conductivity of the coating.
As an alternative to metallic bi-polar plates, graphite or graphite-based composites can be employed as materials for making bi-polar plates. Graphite exhibits good corrosion resistance in fuel cell environments. Graphite bi-polar plates are typically mechanically machined, whereas composite graphite/polymer plates are typically manufactured by molding or embossing. The disadvantages of the graphite-based plates are that the methods of making the plates are expensive, the plates lack mechanical strength, and the plates are difficult to manufacture. The weak mechanical properties of graphite bi-polar plates require thicker plates vs. metallic plates, which results in an increase of the overall size of the fuel cell. In addition, thermal and electrical conductivity of graphite bi-polar plates are inferior to the thermal and electrical conductivity of metallic bi-polar plates, resulting in additional electric losses and overheating.